1. Field of the Invention
The present invention relates to a power semiconductor device having a current detecting function and a method of producing the same.
2. Description of the Related Art
The power semiconductor device is a device designed to let flow currents of power levels such as those for driving motors to monitor conduction currents and to check for overcurrents. This requires the device to detect currents flowing therethrough or flowing through a load circuit connected thereto. There have been proposed a number of devices having a current detecting function each.
One such device is disclosed in U.S. Pat. No. 4,783,690. This is a power semiconductor device comprising a plurality of vertical double-diffused insulated gate transistors (DMOS cells) each having two kinds of well regions: a deep well region not wide enough to reach under a gate electrode and intended primarily to prevent a punch-through at a source region, and a channel well region shallower than the above well region and reaching a channel region immediately under the gate electrode to determine the threshold voltage of that channel region. The disclosed device has a principal current part having a plurality of integrated DMOS cells, and a current detection part having likewise a plurality of integrated DMOS cells and branching part of the current flowing through the device for current detection. The principal current part and the current detection part share the substrate as their drain. Gate electrodes are fed with the same potential, and source regions are incorporated in different channel regions to divide the source current therebetween. Because the DMOS cells of the principal current part and of the current detection part are formed in the same shape on the same chip using the same process, the current flowing through the principal current part is estimated using a source follower resistance to detect the current flowing through the DMOS cells constituting the current detection part.
When a power semiconductor device is in use, its drain electrode is vulnerable to a surge voltage as well as to a fly-back voltage from driving under reactance load. The device is protected from such overvoltages generally by suitable means for breaking down the parasitic diode formed by a PN junction between the drain and well regions. In this manner, the voltage applied to the power semiconductor device is fixed in a safe operating range of the device.
One disadvantage of the disclosed semiconductor device having current detecting function is a possible destruction of DMOS cells in its current detection part. The destruction can occur as follows: because the well region of the current detection part and that of the principal current part are formed in the same shape using the same process, the vertical reach-through voltage of one region is the same as that of the other region. That is, a reach-through occurring in the principal current part will likely entail a concurrent reach-through in the current detection part. Hence the possible destruction of DMOS cells in the current detection part having a lower current capacity than the DMOS cells in the principal current part.